There are two Global Navigation Satellite Systems (GNSS) which have been fully deployed for a number of years (the US Global Positioning System, the Russian GLONASS) and two more which are under deployment (the Chinese Beidou Navigation Satellite System and the European Galileo system). These systems rely on the same principles: microwave radio signals are broadcasted from a number of satellites which orbit on a non-geostationary orbit; the signals carry a code which is correlated with a local replica in a receiver configured to receive the broadcasted signals; when a receiver is capable of acquiring and tracking signals from a minimum number of satellites (generally four), it is able to calculate its own position, velocity, time (PVT) from the pseudo-ranges of the satellites in view.
The precision of the pseudo-ranges is significantly affected by the impact of the course of the radio signals through the atmosphere (ionosphere and troposphere) which can generate an error of more than 10 m. All GNSS offer two signals on two different frequencies, which are used to eliminate the atmospheric errors and perform Precise Point Positioning.
The precision of the pseudo-ranges is also greatly enhanced by using not only the standard code measurements, but also by using the carrier phase measurements. Code measurements have a standard precision (without error corrections) of about 10 m. Carrier phase measurements can yield a cm level precision. But carrier phase measurement is ambiguous by nature, i.e. the number of cycles of carrier signal at the time of measurement is not determined easily from the raw signal of a single carrier. This is notably because the carrier phase ambiguities are impacted by receiver and satellite biases.
Techniques have been developed over time to resolve these ambiguities.
Methods of ambiguity resolution of a first type rely on a Single Difference of observations of signals received by a reference receiver from two different satellites, which eliminate the satellites biases, or from a single satellite at two different reference receivers, which eliminate the reference receivers biases.
Methods of ambiguity resolution of a second type rely on a Double Difference of observations of signals received by two reference receivers from two different satellites, which eliminate both the satellites and reference receivers biases.
Both methods of the first and the second types process the signals of each frequency independently.
Methods of ambiguity resolution of a third type rely on a Zero Difference of observations of signals received by all reference receivers from all satellites in view. The ambiguity resolution of this third type relies on a direct process of calculation of the biases of carrier phase signals, without calculating differences. A method of this third type is disclosed by European patent no EP2140285 assigned to the applicant of the present application. But these methods still rely on a combination of the phase biases in time group delays.
The methods of the third type work well but are implementation dependent. Also, their complexity is significantly increased for triple frequency signals because of the number of possible combinations of signals (from four to nine, in theory).
There is therefore a need for a method to resolve the carrier phase ambiguities without combining the phase biases of the signals.